Conventionally, various contrivances have been made for improving the on-ice performance and on-snow performance of winter tires.
For example, a technology which increases edge components present in a contact patch and simultaneously improves the snow-gripping effect and the travelling performance on icy and snowy road surfaces (iced road surfaces and snowed road surfaces) of tires, by providing a plurality of sipes on each block formed on a tread portion, is proposed in JP 2002-192914 A (PTL 1).
In addition, for example, JP H11-301217 A (PTL 2) proposes a technology which uses foamed rubber as a cap rubber on a tire having a tread rubber with a so-called cap-and-base structure comprising a cap rubber and a base rubber, thereby significantly improving the drainage performance and the on-ice performance and on-snow performance of the tire.
Also, for example, JP 2009-67378 A (PTL 3) proposes, as is shown in FIG. 1(a), for the surface properties of the tread portion 1 of the tire, a technology which increases the roughness of the surface, increases the frictional force between the tire surface and a road surface, and improves the on-ice performance and on-snow performance of the tire, by providing protrusions 2 with a sharp-ended shape on the surface of the tread portion.